Laser spot hardening

ABSTRACT

A hardened/stiffened steel component includes a steel substrate having a bulk region and a steel substrate surface. Plurality of localized martensite-enriched regions is located at the steel substrate surface. Characteristically, the plurality of localized martensite-enriched regions has higher martensite content than the bulk region. Centers of adjacent localized martensite-enriched regions in the plurality of martensite-enriched localized regions are separated by a separation distance that is from about 0.5 to 5 mm. A method for forming the steel component by laser spot hardening is also provided.

TECHNICAL FIELD

In at least one aspect, the present invention is related to steel components that are laser spot hardened.

BACKGROUND

It is known that motor vehicles have a large number of metal and, in particular, steel components. Such components are used because of the easy formability of metal into automobile parts and the strength provided. However, automobile manufacturers are under pressure to reduce a vehicles weight in order to improve fuel efficiency. Therefore, reducing the thickness of metal components is desirable. However, a thickness reduction can lower the rigidity of such components.

Accordingly, there is a need for steel components that maintain rigidity while also decreasing weight.

SUMMARY

The present invention solves one or more problems of the prior art by providing in at least one embodiment, a steel component with improved stiffness and hardness. The steel component includes a steel substrate having a bulk region and a steel substrate surface. Plurality of localized martensite-enriched regions is located at the steel substrate surface. Characteristically, the plurality of localized martensite-enriched regions has higher martensite content than the bulk region. Centers of adjacent localized martensite-enriched regions in the plurality of martensite-enriched localized regions are separated by a separation distance that is from about 0.5 to 5 mm.

In another embodiment, a method for forming the hardened/stiffened steel component set forth above is provided. The method includes a step of providing a steel substrate having a bulk region and a steel substrate surface. The steel substrate is laser spot hardened at a plurality of locations on the steel substrate surface with a laser welding system that includes a laser to form a plurality of localized martensite-enriched regions located at the steel substrate surface. The plurality of localized martensite-enriched regions has higher martensite content than the bulk region. Centers of adjacent localized martensite-enriched regions in the plurality of localized martensite-enriched regions are separated by a separation distance that is from about 0.5 to 5 mm. Advantageously, the increase or decrease of the components strength and stiffness is controlled by the distribution and the size of the laser spot weld. Automotive seat structures or components exhibit predictable and repeatable load path performance with zero increase in mass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A provides a top view of a steel component having a plurality of localized martensite-enriched regions arranged in a rectangular array;

FIG. 1B is a top view of a plurality of localized martensite-enriched regions arranged in a hexagonal array;

FIG. 1C is a side view of a steel component having a plurality of localized martensite-enriched regions;

FIG. 2 is a schematic illustration of a spot laser hardening system for forming a plurality of localized martensite-enriched regions;

FIG. 3A is a perspective view of a vehicle seat frame having steel components;

FIG. 3B is a side view of a steel vehicle seatback frame side component that is stiffened by a plurality of localized martensite-enriched regions;

FIG. 4 is a schematic illustration of a three point bend test system for evaluating the rigidity of steel components; and

FIG. 5 is a plot of three point bend test deflection versus applied normal load.

DETAILED DESCRIPTION

Reference will now be made in detail to presently preferred compositions, embodiments and methods of the present invention, which constitute the best modes of practicing the invention presently known to the inventors. The Figures are not necessarily to scale. However, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various and alternative forms. Therefore, specific details disclosed herein are not to be interpreted as limiting, but merely as a representative basis for any aspect of the invention and/or as a representative basis for teaching one skilled in the art to variously employ the present invention.

Except in the examples, or where otherwise expressly indicated, all numerical quantities in this description indicating amounts of material or conditions of reaction and/or use are to be understood as modified by the word “about” in describing the broadest scope of the invention. Practice within the numerical limits stated is generally preferred. Also, unless expressly stated to the contrary: the description of a group or class of materials as suitable or preferred for a given purpose in connection with the invention implies that mixtures of any two or more of the members of the group or class are equally suitable or preferred; description of constituents in chemical terms refers to the constituents at the time of addition to any combination specified in the description, and does not necessarily preclude chemical interactions among the constituents of a mixture once mixed; the first definition of an acronym or other abbreviation applies to all subsequent uses herein of the same abbreviation and applies mutatis mutandis to normal grammatical variations of the initially defined abbreviation; and, unless expressly stated to the contrary, measurement of a property is determined by the same technique as previously or later referenced for the same property.

It is also to be understood that this invention is not limited to the specific embodiments and methods described below, as specific components and/or conditions may, of course, vary. Furthermore, the terminology used herein is used only for the purpose of describing particular embodiments of the present invention and is not intended to be limiting in any way.

It must also be noted that, as used in the specification and the appended claims, the singular form “a,” “an,” and “the” comprise plural referents unless the context clearly indicates otherwise. For example, reference to a component in the singular is intended to comprise a plurality of components.

The term “comprising” is synonymous with “including,” “having,” “containing,” or “characterized by.” These terms are inclusive and open-ended and do not exclude additional, unrecited elements or method steps.

The phrase “consisting of” excludes any element, step, or ingredient not specified in the claim. When this phrase appears in a clause of the body of a claim, rather than immediately following the preamble, it limits only the element set forth in that clause; other elements are not excluded from the claim as a whole.

The phrase “consisting essentially of” limits the scope of a claim to the specified materials or steps, plus those that do not materially affect the basic and novel characteristic(s) of the claimed subject matter.

With respect to the terms “comprising,” “consisting of,” and “consisting essentially of,” where one of these three terms is used herein, the presently disclosed and claimed subject matter can include the use of either of the other two terms.

With reference to FIGS. 1A, 1B, and 1C, a steel component with improved stiffness and hardness is provided. Steel component 10 includes steel substrate 12 which includes bulk region 14 and steel substrate surface 16. Examples of suitable steel substrates include, but are not limited to, multiphase steel, dual phase steel, and high-strength low-alloy (HSLA) grades of steel. Typically, steel components will have a thickness from about 0.25 to 3 mm. Plurality of localized martensite-enriched regions 18 are located at the steel substrate surface 16. Characteristically, the plurality of localized martensite-enriched regions 18 has higher martensite content than the bulk region 14. In a variation, steel component 10 also includes an opposite surface 20 which includes a second plurality of localized martensite-enriched regions 22. Centers 26 of adjacent localized martensite-enriched regions in the plurality of martensite-enriched localized regions 18 or 22 are separated by a separation distance d₁. In a refinement, a separation distance d₁ is from about 0.5 to 5 mm. In another refinement, the separation distance d₁ is from about 1 to 4 mm. In still another refinement, the separation distance d₁ is from about 1.5 to 3 mm.

Still referring to FIGS. 1A, 1B, and 1C, various arrangements of the plurality of martensite-enriched localized regions are schematically depicted. FIG. 1A illustrates a rectangular array in which rows 28 of at least a portion of martensite-enriched localized regions 18 are aligned. FIG. 1B illustrates a hexagonal array in which rows 28 of at least a portion of martensite-enriched localized regions 18 are offset. Each localized martensite-enriched region 18 has a maximum spatial extent d₂ parallel to the steel substrate surface 16. In a refinement, maximum spatial extent parallel d₂ is less than or equal to 5 mm. In another refinement, maximum spatial extent d₂ is less than or equal to 4 mm. In other refinements, the maximum spatial extent d₂ is less than or equal to 3 mm. Typically in these refinements, the maximum spatial extent parallel d₂ is greater than or equal to 1 mm. The localized martensite-enriched regions 18 extend below the steel substrate surface to a depth d₃. In a refinement, the depth d₃ is from about 50 to 600 micrometers. In another refinement, the depth d₃ is from about 150 to 400 micrometers. In still another refinement, the depth d₃ is about 300 micrometers.

The steel component of the present embodiment is advantageously stiffer and more rigid than a steel component of the same bulk region composition without having a plurality of localized martensite-enriched regions 18. In this regard, the localized martensite-enriched regions 18 have an average Rockwell hardness value from about 35 to 60 HRC as measured on the C scale and the bulk region has an average Rockwell hardness value less 35 HRC as measured on the C scale. In a refinement, the steel component has a deflection that is at most 50 percent of the deflection of a steel component (of the same type and dimensions) without the localized martensite-enriched regions for a 1 mm thick sample as determined by the three point bend tests set forth below with a load of 300 N. In another refinement, the steel component has a deflection that is at most 40 percent of the deflection of a steel component (of the same type and dimensions) without the localized martensite-enriched regions for a 1 mm thick sample as determined by the three point bend tests set forth below with a load of 300 N. In still another refinement, the steel component has a deflection that is from 20 to 50 percent of the deflection of a steel component (of the same type and dimensions) without the localized martensite-enriched regions for a 1 mm thick sample as determined by the three point bend tests set forth below with a load of 300 N.

With reference to FIG. 2, a method for hardening and/or stiffening a steel substrate to form the steel component set forth above is provided. Steel substrate 12 includes a bulk region 14 and a steel substrate surface 16. The details for the steel substrate 12 are set forth above. Steel substrate 12 is subjected to laser spot hardening at a plurality of locations 30 on the steel substrate surface 16 with laser welding system 32. Laser welding system 32 includes a laser 34 to form the plurality of localized martensite-enriched regions 18 located at the steel substrate surface. Laser welding system 32 also includes galvo systems 38, 40 for adjusting the direction of laser beam 42 and lens 44 for focusing laser beam 42. Galvo systems 38, 40 include mirrors 42, 44 respectively that can be used to steer laser beam 42 along directions d₄ and d₅ to form the plurality of localized martensite-enriched regions 18 and/or 22. Galvo systems 38, 40 can rotate mirrors 44, 46 about angles a₁ and a₂. In a refinement, laser 24 melts the steel within about 5 ms which upon solidification creates a martensitic structure in the steel. In a refinement, laser welding system 32 includes a fiber laser. Although the present invention is not limited by the laser power, typically laser 34 has a power output from about 0.1 to 3 MW/cm². The light from the laser 34 is sequentially directed to the plurality of locations 30 on the steel substrate surface via galvo systems 38, 40. In a refinement, steel substrate 12 is sequentially moved to direct light onto a plurality of locations on the steel substrate surface.

FIGS. 3A and 3B provides schematic illustrations of a vehicle seat frame that includes steel components that can be stiffened/hardened. Vehicle seat frame 50 includes seatback frame 52. Seatback frame 52 includes seatback side members 54, 56 attached to seatback cross members 58, 60. Vehicle seat frame 50 also includes a seat bottom frame 62 positioned below seatback frame 52. Typically, one or more of these vehicle seat frame components are made of steel which is suitable for laser hardening as set forth above. For example, FIG. 2B shows seatback side member 54 which includes several distinct regions (can be 1, 2, 3, 4, 5 or more) each having a plurality of localized martensite-enriched regions 64-70 formed by laser spot hardening. The localized martensite-enriched regions illustrated in this disclosure are generally circular, though they need not be.

With reference to FIG. 4, a system for testing rigidity of the steel components is provided. Three point bend testing system 80 includes side supports 82, 84 for suspending a steel component 10. A normal force F is applied at the center of steel component 10 and the amount of deflection d₆ measured. FIG. 5 provides a plot of the deflection for several separations of the localized martensite-enriched regions for a 780DP steel sample having a thickness of 1.0 mm. For these samples, there are higher stiffness properties after heat treatment than the base material. For a sample having laser spots on a single side, the force is applied to the opposite side. The three point bend test showed a 47% decrease in extension from 3.9 mm on the base material to 2.1 mm for the 1.5 mm spacing sample at 421 N. Table 1 provides values of the deflection for several grades of steel. Generally, the laser treated steel samples exhibit less deflection under load.

TABLE 1 Three point deflection values. Sample ID Extension at load Load(N) Δ% 340 XF Steel Baseline 4.495 257.2 61.4% 340 XF Steel Spacing 1.733 1.5 mm, Side B 420 XF Steel Baseline 4.173 264.4 62.8% 420 XF Steel Spacing 1.557 1.5 mm, Side B 490 XF Steel Baseline 4.437 308.8 59.8% 490 XF Steel Spacing 1.782 1.5 mm, Side B 780 DP Steel Baseline 3.863 421.3 46.6% 780 DP Steel Spacing 2.064 1.5 mm, Side B 980 DP Steel Baseline 6.202 530.2 40.7% 980 DP Steel Spacing 3.678 1.5 mm, Side B 1180 DP Steel Baseline 7.327 623.3 33.6% 1180 DP Steel Spacing 4.865 1.5 mm, Side B

Table 2 provides values of the ultimate tensile strength (UTS) for the base material and the laser spots for several steel samples. In general, the UTS is increased by over 100 percent.

TABLE 2 UTS strength for steel samples Base Mat'l Laser Spot Sample Material Strength (UTS) Strength (UTS) % Increase Number Grade (MPa) (MPa) (%) 1 340XF 565 1296 129 2 420XF 593 1220 106 3 490XF 634 1048 65 4 780DP 807 1386 72 5 980DP 1007 1758 75 6 1180DP  1179 1951 65 *The above UTS strengths are based from the measured micro-hardness results in the base material and the laser spot hardened zone.

Finally, Table 3 provides micro hardness values in the laser spot harden zone at several distances from the steel surface. In general, the micro hardness was higher than the value of the base material (i.e., the bulk region).

TABLE 3 Micro Hardness Values Location from Surface Hardness Value 0.050 mm 40 HRC 0.100 mm 38 HRC 0.200 mm 40 HRC HAZ (heat-affected zone) 27 HRC Base Material 28 HRC

While exemplary embodiments are described above, it is not intended that these embodiments describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention. Additionally, the features of various implementing embodiments may be combined to form further embodiments of the invention. 

What is claimed is:
 1. A steel component comprising: a steel substrate having a bulk region and a steel substrate surface; and a plurality of localized martensite-enriched regions located at the steel substrate surface, the plurality of localized martensite-enriched regions having higher martensite content than the bulk region, wherein centers of adjacent localized martensite-enriched regions in the plurality of martensite-enriched localized region are separated by a separation distance that is from about 0.5 to 5 mm.
 2. The steel component of claim 1 wherein the separation distance is from about 1 to 4 mm.
 3. The steel component of claim 1 wherein the separation distance is from about 1.5 to 3 mm.
 4. The steel component of claim 1 wherein at least a portion of the plurality of martensite-enriched localized regions are arranged in a rectangular array.
 5. The steel component of claim 1 wherein at least a portion of the plurality of martensite-enriched localized regions are arranged in a hexagonal array.
 6. The steel component of claim 1 wherein each localized martensite-enriched region has a maximum spatial extent parallel to the steel substrate surface less than 4 mm.
 7. The steel component of claim 1 wherein the steel substrate comprises a multiphase steel or dual phase steel.
 8. The steel component of claim 1 wherein the localized martensite-enriched regions have a maximum spatial extent parallel to the steel substrate surface less than 3 mm.
 9. The steel component of claim 1 wherein the localized martensite-enriched regions have an average Rockwell hardness value from about 35 to 60 HRC as measured on the C scale and the bulk region has an average Rockwell hardness value less 35 HRC as measured on the C scale.
 10. The steel component of claim 1 wherein the localized martensite-enriched regions extend below the steel substrate surface to a depth from about 50 to 600 micrometers.
 11. A method for hardening a steel component comprising: providing a steel substrate having a bulk region and a steel substrate surface; and laser spot hardening the steel substrate at a plurality of locations on the steel substrate surface with a laser welding system that includes a laser to form a plurality of localized martensite-enriched regions located at the steel substrate surface, the plurality of localized martensite-enriched regions having higher martensite contents than the bulk region, wherein centers of adjacent localized martensite-enriched regions in the plurality of localized martensite-enriched region are separated by a separation distance that is from about 0.5 to 5 mm.
 12. The method of claim 11 wherein the laser melts the steel substrate within 5 ms which upon solidification creates the plurality of localized martensite-enriched regions.
 13. The method of claim 11 wherein the laser welding system includes a fiber laser.
 14. The method of claim 11 wherein the laser has a power output from about 0.1 to 3 MW/cm2.
 15. The method of claim 11 wherein light from the laser is sequentially directed to the plurality of locations on the steel substrate surface.
 16. The method of claim 11 wherein the steel substrate is sequentially moved to direct light onto a plurality of locations on the steel substrate surface.
 17. The method of claim 11 wherein the separation distance is from about 1 to 4 mm.
 18. The method of claim 11 wherein the separation distance is from about 1.5 to 3 mm.
 19. The method of claim 11 wherein at least a portion of the plurality of martensite-enriched localized regions are arranged in a rectangular array or a hexagonal array.
 20. The method of claim 11 wherein the steel substrate comprises a multiphase steel, dual phase and HSLA grades of steel. 